Filter display_post_states to show 'Liveblog' in edit post table

See Automattic/liveblog#55
Add select to filter posts list table by liveblog state

See Automattic/liveblog#55
Add explicit check for is_liveblog_post in display_post_state filter

The is_liveblog_post() method actually just uses get_liveblog_state(), but this is more clear.

See https://github.com/x-team/wp-liveblog/commit/68e2c7bd58f3ca952558ac074f48ba5b4f3e118b#commitcomment-3570526
Add is_main_query() check for liveblog_state filtering pre_get_posts action

See https://github.com/x-team/wp-liveblog/commit/87fa964cb277c8de14445729ca8601067fbaa4c5#commitcomment-3570531
Add liveblog_state query_var only in admin

See https://github.com/x-team/wp-liveblog/commit/87fa964cb277c8de14445729ca8601067fbaa4c5#commitcomment-3570540
Improve method names

See https://github.com/x-team/wp-liveblog/commit/87fa964cb277c8de14445729ca8601067fbaa4c5#commitcomment-3570551
Simplify meta_query appending logic in pre_get_posts action

See https://github.com/x-team/wp-liveblog/commit/87fa964cb277c8de14445729ca8601067fbaa4c5#commitcomment-3570537
Merge pull request #101 from x-team/issue-55

Issue 55: Filter Liveblog entries on Dashboard Edit Screen